Depending on the individual in question, different methods may be appropriate. The most accurate is to actually work up to the heaviest single rep you can. If this is contraindicated for any reason, 1RMs can be estimated based on loading used for multiple reps.

To determine an actual 1RM lift, make sure you're recovered from any prior training and properly warmed up. Some specific warm-up activity will be helpful, but keep the volume very low to prevent overtaxing. Start with a load you know you can lift once, and lift it once. Based on how it feels, add weight and perform another single after 3-5 minutes of rest. Repeat this process until you reach what appears to be your maximum effort. Try to make big enough jumps between sets that you can reach your 1RM in 5 sets or less to prevent limiting fatigue.

To estimate the 1RM based on a multiple rep load, there are charts available that are fairly consistent among sources. Here are a few numbers you can use:

1 rep = 100%
5 reps = 87%
10 reps = 75%
15 reps = 65%

Remember, these multiple reps are for a single set; that is, 10RM is as much as you can do for only a single set. Don't use rep numbers from multiple sets.